Measurement of neutrons has been of interest since the development of nuclear power. Assay of remote handled radioactive waste for transuranic content is becoming more important as the cleanup of radioactive waste progresses. Remote handled radioactive waste is defined as radioactive waste having more than 100 nCi/g of radioisotopes heavier than .sup.238 U and with half lives greater than 20 years. Assay for transuranic content can be made by detecting neutrons and inferring transuranic (plutonium) content. Present equipment for measuring neutrons using He.sup.3 proportional counters ceases to accurately function at gamma-ray exposure rates of about 4 R/hr and above. High gamma-ray exposure rates are often present from transuranic waste because of the presence of fission products. By sacrificing sensitivity, measurements may be obtained with exposures from 10-50 R/hr with existing systems. Because remote handled waste has a minimum threshold of a contact exposure rate of 200 mR/hr and can be as high as 10,000 R/hr, existing instruments are useful only for the lowest exposure wastes.
Hence, there is a need for a neutron detector that can measure neutrons from a waste source having the maximum contact exposure.